Garnet solid solutions have been extensively studied in order to relate macroscopic behaviour with atomic-scale properties deriving from chemical substitutions. Strongest deviations from ideal behaviour are observed in the pyrope (prp)-grossular (grs) join, where the presence and extent of a compositional solvus have also been debated. We present single-crystal refinements of garnets with grs content ranging from 22% to 89%, which were found in metarodingites from Cima di Gagnone (Canton Ticino, Switzerland). Many crystals have compositions close to prp50grs50, and thus fall into the region of maximum deviation from ideal behaviour, and at the prp side of the miscibility gap observed in natural garnets (UNGARETTI et al., 1995). A number of geometric parameters increasingly depart from linear behaviour when the grs content increases from 0 to 50%. Also the measured values of the adp's at the Y, Z, and O sites as well as their shape, show non-ideal behaviour and are coherent with increasing structural strain. Structure modelling allowed to correct the measured unit-cell volumes for the concentrations of chemical constituents different from those of prp and grs (i.e., for the other end-member components) and to provide new values for the ?Vmix for the prp-grs join. They are generally larger than those inferred from previous work based on powder analysis of synthetic crystals, and also define a different trend for non-ideal behaviour (Figure 1). All these evidences are coherent with a very peculiar structural behaviour at prp50grs50, which does not correspond to a linear combination of the structures of the two end members. This composition should be less stable than prp and grs, which justifies its rarity in geologic environments. Accordingly, peculiar petrogenetic conditions have been inferred from field evidences at Cima di Gagnone (P~3 GPa, T~800 °C; PFIFFNER & TROMMSDORFF, 1998). No change in symmetry could be observed, and thus prp50grs50 cannot be considered a new garnet end-member in the present nomenclature scheme. However, it should be considered as a further component when modelling thermodynamic behaviour of garnets solid solutions. PFIFFNER, M., & TROMMSDORFF, V. (1998). Schweiz. Mineral. Petrogr. Mitt., 78: 337-354. UNGARETTI, L., LEONA, M., MERLI, M., & OBERTI, R. (1995). Eur. J. Mineral., 7: 1299-1312.
The pyrope-grossular solid solution: new crystal-chemical data to model non-ideal behaviour
OBERTI R
2001
Abstract
Garnet solid solutions have been extensively studied in order to relate macroscopic behaviour with atomic-scale properties deriving from chemical substitutions. Strongest deviations from ideal behaviour are observed in the pyrope (prp)-grossular (grs) join, where the presence and extent of a compositional solvus have also been debated. We present single-crystal refinements of garnets with grs content ranging from 22% to 89%, which were found in metarodingites from Cima di Gagnone (Canton Ticino, Switzerland). Many crystals have compositions close to prp50grs50, and thus fall into the region of maximum deviation from ideal behaviour, and at the prp side of the miscibility gap observed in natural garnets (UNGARETTI et al., 1995). A number of geometric parameters increasingly depart from linear behaviour when the grs content increases from 0 to 50%. Also the measured values of the adp's at the Y, Z, and O sites as well as their shape, show non-ideal behaviour and are coherent with increasing structural strain. Structure modelling allowed to correct the measured unit-cell volumes for the concentrations of chemical constituents different from those of prp and grs (i.e., for the other end-member components) and to provide new values for the ?Vmix for the prp-grs join. They are generally larger than those inferred from previous work based on powder analysis of synthetic crystals, and also define a different trend for non-ideal behaviour (Figure 1). All these evidences are coherent with a very peculiar structural behaviour at prp50grs50, which does not correspond to a linear combination of the structures of the two end members. This composition should be less stable than prp and grs, which justifies its rarity in geologic environments. Accordingly, peculiar petrogenetic conditions have been inferred from field evidences at Cima di Gagnone (P~3 GPa, T~800 °C; PFIFFNER & TROMMSDORFF, 1998). No change in symmetry could be observed, and thus prp50grs50 cannot be considered a new garnet end-member in the present nomenclature scheme. However, it should be considered as a further component when modelling thermodynamic behaviour of garnets solid solutions. PFIFFNER, M., & TROMMSDORFF, V. (1998). Schweiz. Mineral. Petrogr. Mitt., 78: 337-354. UNGARETTI, L., LEONA, M., MERLI, M., & OBERTI, R. (1995). Eur. J. Mineral., 7: 1299-1312.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
